1. Field of the Invention
The present invention relates to an active type harmonic suppression apparatus. More particularly, the present invention relates to the active type harmonic suppression apparatus connected in parallel to a power feeder for filtering harmonic currents thereof.
2. Description of the Related Art
Recently, the controllable power electronic devices suitable for high voltage rating and large current rating characteristics have been developed due to the improvement of the semiconductor manufacturing technique. Power electronic devices are widely applied in electric power equipment, such as motor driver, arc furnace, trolley car, battery charger, and lighting appliance etc. These electric power equipment may generate a large amount of harmonic currents due to the nonlinear input characteristic of such loads. The harmonic currents may result in the problems, such as transformer overheat, rotary machine vibration, voltage quality degradation, electric power components damage, medical facilities malfunction etc. In order to improve the problems of harmonic pollution effectively, many harmonic control standards, such as IEEE519-1992, IEC1000-3-2, and IEC1000-3-4 etc., have been established. In Taiwan, there is also established a power control provisional standard for harmonic by Taiwan Power Company.
Conventionally, the passive power filter is used to solve the problems of harmonic. The passive power filter generally consists of inductor and AC capacitor. In general, the passive power filter may cause the resonance problem that may generate larger harmonic current and voltage. The resonance of the passive power filter may damage the passive power filter itself and neighboring power facilities. Additionally, it is quite hard to obtain a better filtering effect of the passive power filter due to the system reactance that may easily affect the filtering characteristic of the passive power filter. Furthermore, the harmonic current of neighboring non-linear loads may flow into the passive power filter and result in overload of the passive power filter.
Currently, a power converter consisting of power electronic devices is carried out for filtering harmonics and disclosed in the art, such as U.S. Pat. Nos. 6,472,775 and 6,320,392.
Referring initially to FIG. 1, a schematic circuitry of a conventional active power filter is illustrated. In the illustrated circuitry, the active power filter includes a dc capacitor, a power converter and a filter element. The power converter has a current-mode control controlled to generate a compensating current via the filter element to be injected into a power feeder. The filter element is selected from an inductor or a combination of an inductor and a capacitor. The filter element is used to filter high-frequency ripple current due to the switching operation of power converter. Furthermore, in order to filter the high-frequency ripple current effectively, the selection of filter element depends on switching frequency, dc voltage of the power converter and the limitation of ripple current.
Although the active power filter performs a better filtering characteristic, the capacity of the power converter in an active power filter must be greater than the product of the harmonic current of the load and the voltage of the power source. Therefore, the manufacture cost of the power converter of the active power filter must be significantly increased so that applications of the active power filter are disadvantageously limited. Generally, the filter element of the active power filter, having lower impedance under the fundamental frequency, is only used to filter ripple current, and thus voltage of the dc capacitor must be greater than the voltage peak of the power source. However, a higher voltage of the dc capacitor may result in generating a higher ripple current. Accordingly, the filter element must employ a low-pass filter with a lower cut-off frequency for filtering higher switching ripple currents. This results in the poor transient response and an increase of overall dimensions, and further results in higher electromagnetic interference.
A hybrid power filter structure consisting of a passive power filter and a power converter is developed to overcome the capacity and cost problems caused by the power converter of the active power filter because the passive power filter is used to reduce the capacity of the power converter. For example, the hybrid power filter structure is disclosed in U.S. Pat. Nos. 5,567,994 and 5,731,965. However, these hybrid power filters employs a passive power filter having a larger inductor that may results in an increase of dimensions. Another hybrid power filter structure, such as U.S. Pat. Nos. 6,717,465 and 5,614,770, employs a capacitor which may minimize dimensions and reduce power loss. In U.S. Pat. No. 5,614,770, the hybrid power filter adopts a current-mode control. A compensation current is calculated and controlled with a feedback output current of the power converter in close loop control so that the power converter acts as a current generator. The current generator may generate a compensation current that is injected into a power feeder for filtering harmonic currents. However, the power converter with a current-mode control is limited by bandwidth and gain of the controller such that a specific error between the output current of the power converter and the calculated compensation current may be existed. In addition, the current of the power source may contain a small distortion after compensation because no power source current is detected and feedback to the controller. In U.S. Pat. No. 6,717,465, the hybrid power filter adopts a voltage-mode control. The power converter thus acts as a voltage source, and the power source current is detected and feedback to the controller, so that current waveform distortion of the power source can be reduced. In this way, the current waveform distortion problem due to no power source current feedback disclosed in U.S. Pat. No. 5,614,770 is solved. However, there is a difficulty for manufacturing the power converter disclosed in U.S. Pat. No. 6,717,465 due to pre-evaluation the value of passive elements serially connected with the power converter. In addition, it may perform poor filtering characteristics since the value of the passive element may be shifted under high temperature conditions and deterioration.
The present invention intends to provide an active type harmonic suppression apparatus so as to improve an evaluation problem for the value of the passive elements serially connected with the power converter with a voltage-mode control disclosed in U.S. Pat. No. 6,717,465. Accordingly, the hybrid power filter can be manufactured easily, and a filtering effect thereof may not be affected by deterioration of the passive elements. The present invention controls harmonic components of the output currents of the power converter and those of the load currents in a close loop control circuit so as to improve a voltage controller with a voltage-mode control, disclosed in U.S. Pat. No. 6,717,465, sensitive to the changed values of the passive elements serially connected with the power converter. The present invention can carry out a simplified control circuit block for the harmonic suppression apparatus and adjust a reactive power current supplied from the harmonic suppression apparatus.